This invention relates to the field of communications, and in particular to a base station that communicates with a plurality of remote devices using a message cycle that includes a fixed number of frame slots, and each remote device is configured to communicate at a defined frame rate at one or more frame slots within the message cycle.
A number of opportunities exist for the communication of short but informative messages between a plurality of remote devices and a base station.
A particularly unique application for transmitting short and informative messages from remote devices to a base station includes the “SPOT” system (SPOT Satellite GPS Messenger™, SPOT LLC, a wholly-owned subsidiary of Globalstar Inc.) that is configured to transmit, periodically or on demand, “I'm OK”, or “I need help” messages with GPS coordinates. The base station is configured to relay the “I'm OK” messages with coordinates via e-mails to recipients associated with each particular transmitter, and to relay the “I need help” messages with coordinates in an alert with coordinates to an emergency service associated with the coordinates. In like manner, a low-cost “OnStar”-like system can be deployed on vehicles, such that if an accident or malfunction is sensed, a request for assistance is automatically transmitted. Similarly, location-reporting units can be deployed on taxis, trucks, service vehicles, and the like, to provide dispatchers and others with up to date status information.
Opportunities also exist for the communication of short but informative messages from the base station to the remote devices. Paging systems are classic examples of systems that provide short but informative messages. In like manner, in the aforementioned emergency notification system, for example, an acknowledgement of receipt of each message is desirable to reassure the user that the alarm has been received. Subsequently, periodic updates would also be desirable, providing, for example, the estimated time of arrival of the respondent aid. Similarly, the base station may prompt any active remote device that hasn't reported its position for a substantially long time period, and take appropriate action if a reply is not received. In a vehicle or cargo tracking system, the location transmitter may be configured to report its position when requested by the base station. In this manner, the requests can be managed such that fewer transmissions are requested when the tracked object is detected to be traveling in a remote area, where the potential options for misrouting are few, and more transmissions are requested when the tracked object is within a city. Other opportunities include the remote control of appliances based on messages relayed by users through the base station, and others.
U.S. Pat. Nos. 6,396,819, 6,317,029, 6,985,512, and 6,856,606 present novel techniques for efficiently communicating such short messages between a base station and the remote devices based on the use of a common DSS spreading code and a variety of code-phases at the remote devices, and are incorporated by reference herein. Each remote device independently transmits its message, repeatedly, a number of times. Because the messages are relatively short, and interfering collisions only occurs when the multiple transmissions are concurrent and in phase with each other, a repeated transmission of each message with gaps between the repeated transmissions increases the likelihood that at least one of the multiple copies of the message is properly received at the base station. The likelihood of successful communication can be increased by increasing and varying the number of repeated transmissions, and varying the interval between transmissions. Increasing the number of repetitions of each message may increase the likelihood of successful communication, but it also increases the likelihood of collisions, and as the number of remote devices increases, a saturation point will be reached.
Reception of messages from the base station at the remote devices is not subject to such interfering collisions, because the base station controls when each message is transmitted. However, in many cases, the remote devices are portable units, and the continuous monitoring for independent transmissions from the base station will quickly deplete the batteries in these portable units.
In conventional mobile communication networks, the mobile stations are configured to periodically monitor for notification of messages, and enter an inactive state during periods of inactivity. If a mobile station is notified of a pending message the mobile station is subsequently notified of a time-slot at which the base station will be transmitting the message. A similar protocol is used for allocating time-slots for transmissions. The interval between notifications is generally based on the expected traffic volume and a maximum acceptable lag time between a time at which a message is ready to be transferred from the base station, and the time it is actually transmitted to the receiver, herein termed latency. A long interval between notifications provides a longer inactive period for each remote receiver, conserving battery power, but causes longer latency. A short interval between notifications provides shorter latency, but increases the rate at which a battery will discharge while repeatedly switching to the active state.
A communications service provider using the conventional mobile communication protocols is presented with a dilemma. Should the communications service provide long battery life, or short message delays? In the telecommunications field, that problem is conventionally solved by providing different channels for different modes of operation. Applications with high speed demands, such as voice communications, are handled by one system, while applications with low speed demands, such as pager communications, are handled by another system. In the field of short message communications, a paging application will have substantially higher quality and speed demands than other applications, such as routine vehicle tracking.
In like manner, the provider of a user service, such as a vehicle tracking service, is presented with the dilemma of choosing a provider of high speed or high bandwidth communication to satisfy the requirement of some customers for high resolution (in time or space) vehicle tracking, or a lower-cost provider of low speed or bandwidth for customers who are satisfied knowing the general location of the tracked object during some general time period. This dilemma is further complicated by customers who require different resolutions under different conditions, and expect to pay a service charge that is dependent upon the actual demand/usage of the service. As in the case of the service provider, the traditional solution to a provider of user services that include various, and sometimes varying, levels of demand from its users is to purchase and manage access to multiple systems, each having a particular level of performance, to satisfy the different levels of user demand.
Providing multiple independent systems, however, introduces substantially more overhead than a unified system. For example, each independent system will be configured to handle peak traffic loads, rather than average traffic loads, meaning that much of the system capacity will be unused most of the time. If two independent systems are deployed, the overhead required to accommodate peak traffic loads will be doubled. If a unified system is used, wherein a mix of traffic types are supported, the overhead for accommodating peak traffic loads will only be incurred once. Also, the mix of traffic types is likely to provide a peak traffic load that is less than the sum of the peak traffic loads used in the individual systems, as the different characteristics of traffic types are likely to provide some ‘smoothing’ of the cumulative peak demand. In like manner, the provider of user services could provide various levels of performance to its users, without incurring the costs associated with managing access to multiple systems based on the required performance.
In the field of short message communication, low per-unit cost is a primary criterion, as well as the ongoing cost of operation. It would be advantageous to reduce the ongoing cost of communication by providing a unified system for handling all types of short-message applications on any channel, thereby obviating the need to provide different systems for each application type. It would be advantageous to reduce the per-unit cost by using substantially the same communications circuitry in each unit, regardless of the intended application for the unit. It would also be advantageous to provide this circuitry as a self-contained ‘drop-in’ to any short-message communication application. It would also be advantageous to optimize battery life by consuming power in proportion to the requirements of the application. It would also be advantageous to be able to optimize performance by managing the transmissions from the base station based on operational requirements of the application.
These advantages, and others, can be realized by a method and system that allocates one or more frame slots to each transceiver for communication within each message cycle. The number of frame slots allocated can be dynamically adjusted to accommodate variable traffic loads per transceiver, and an offset of the frame slots within the message cycle is preferably predefined to provide a uniform distribution among the transceivers. The design of the transceiver is independent of the particular application, having at least one programmable parameter that controls the number of frame slots allocated within the message cycle. By controlling the number of frame slots allocated to a transceiver, the amount of inactive time, and hence battery life, can be controlled. When a conflict occurs among multiple transceivers having pending messages at the same frame slot, the allocation of the frame slot to a transceiver is based at least in part on the resultant lag time to each transceiver.
In a preferred embodiment, the frame slots assigned to each remote device are consistent within each message cycle, allowing the determination of the frame slots assigned to each unit based solely on an identification of an offset frame slot within the message cycle assigned to the unit and an identification of the allocated interval between assigned frame slots for that remote device. Preferably, the remote device uses an inherent identifying aspect of itself, such as its serial number, MAC address, etc., which is also known to the base station, to determine its assigned offset frame slot, and the base station defines the interval between listening periods for each remote device, based on the bandwidth allocated to the remote device.
Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention.